In commercial food processing plants, refrigeration demands are oftentimes substantial and varied during processing of the food products, particularly where rapid freezing of the latter is required. Many such plants utilize one or more horizontal contact plate freezer units, each having from ten to thirty plates or refrigerant evaporators between which the product passes during the freezing cycle. U.S. Pat. No. 2,882,697 discloses one type freezer commonly utilized in such plants.
Each plate is subjected to a different refrigeration load which varies depending upon the relative position of the plate within the unit at a given period during the freezing cycle. For example, at the time the warm, unfrozen product enters the freezer unit and is disposed between a pair of contact plates, a large refrigeration load (e.g., 3-6 tons) is imposed on the plates. At the same time certain of the other loaded plates within the unit may be near the end of their respective freezing cycle with the result that the refrigeration load imposed thereon is minimal (e.g., 0-1 ton). As a result of this refrigeration load demand differential, effective and proper control of the refrigeration feed to the various unit plates becomes a difficult and complex problem. In order to circumvent this control problem, many users of contact plate freezers have pumped liquid refrigerant within the system so as to provide an overfeed of approximately eleven or twelve to one (11-12 to 1) with respect to the average refrigerant requirement for the entire system. Because of this overfeed condition, serious problems have developed in that substantial static heads of the liquid refrigerant have resulted in the suction header incorporated in the system. Such heads may seriously impair the operating efficiency of the system. The problem oftentimes is enhanced where the plant in which the system is installed is a single floor structure.
In an effort to alleviate this problem, one or more of the following approaches have been attempted: (a) providing gravity drainage of the liquid refrigerant from the header into a deep, large capacity, recessed collection pit or reservoir and then utilizing a conventional pump to transfer the accumulated liquid refrigerant to an overhead suction main; (b) providing gravity drainage into small capacity on-floor accumulators and then alternately emptying such accumulators by the imposition of high-pressure vapor obtained from the high side of the refrigeration system; and (c) in place of a liquid refrigerant such as ammonia, which has a high latent heat value, circulating at an excess overfeed rate (e.g., one hundred to one) live brine.
It has been found, however, that each of these approaches is beset with one or more of the following shortcomings: (1) the operating efficiency of the system is extremely low; (2) the initial costs for the equipment and installation thereof are inordinately high; (3) the maintenance and service costs of the various components of the system are also inordinately high; (4) a wide variety of freezer types cannot be readily incorporated in the system; and (5) substantial floor space is required to accommodate the system components.